1. Field of the Invention
The present invention relates to the field of insertable or implantable materials or devices in which the material or device is secured into the tissue of a patient through a helical or screw element which is secured into tissue or the like. In particular, the present invention relates to protective elements such as protective caps over a penetrating or pointed section of the material or device, wherein the protective element is capable of timely removal (as by dissolution) from the penetrating or pointed section during technical (e.g., medical) procedures.
2. Background of the Art
Many therapeutic or protective procedures for patients include the implantation of devices into a patient. Such implantations include drug delivery systems, electrostimulating devices (such as pacemakers or pain reduction devices), monitoring devices, electrical leads, electrodes, sensor elements, etc. These devices often have to be firmly secured within the patient to prevent movement of the device that would defeat or diminish its effectiveness. This is particularly true with electrical leads in pacing or defibrillation devices, which must be precisely located so that patient monitoring and electrical stimulation is effective. There are a number of different formats for the securement of electrical leads in patients, including, but not limited to, clips, sutured attachment, corkscrew-like inserts (referred to as helical inserts), and other conventional securement formats found in mechanical systems.
A preferred means of securing leads is the helical insert such as found in the Guidant Cardiac Rhythm Management (CRM) Sweet-Tip® Model 4269 bipolar endocardial lead. This lead comprises a helical element having a base side (proximal end) with an electrode and a sharp tip on an insert side (a distal end) of the element. The pointed end penetrates tissue when a rotating motion is applied to the helical element, causing the element to puncture and or screw into the tissue, advancing the proximal end towards the tissue. The proximal end may have a relatively flat or convex electrical plate, electrode, sensing element (e.g., semiconductor, circuit board, pressure plate, etc.) or contact, and the advancing of the helical element into the tissue brings the contact into firm position with the tissue. In pacing or defibrillating devices, the electrical discharge passes through the electrode and/or into the helical connecting element. In some leads, the helical element is coated with a thin insulating coating layer (which must also be biocompatible) to render the helical element inactive or passive (from the standpoint of discharge). Typical coatings could include ceramics, and polymers such as polyamides, polyimides, polyurethanes, silicone resins, polyacrylates, and especially poly-para-xylylene (e.g., Parylene C).
These types of devices may be inserted into a patient by a number of different medical procedures. The less invasive or traumatic the procedure, the more desirable is that procedure. For example, although the electrodes may be inserted by open chest surgery, the delivery of the electrode through catheterization techniques through arteries or veins is much more preferred. The difficulties involved with passing a sharp element through the vasculature of a patient can be readily appreciated, especially where the path can be tortuous. Further difficulties in passing a sharp element arise when medical procedures require passing the element through heart valve structures. To avoid damage to the patient, the Guidant CRM Sweet-Tip® Model 4269 bipolar endocardial lead provides a mannitol cap over the helical element in the lead. The mannitol cap provides a protective cover for the helical element which prevents the point of the helical element from scraping or puncturing interior walls of the vasculature, valve leaflets or other tissue during introduction of the element to the patient. The mannitol effectively dissolves during the procedure, depending on the placement of the electrode and other environmental factors, usually over the course of about 4 to 10 minutes. This practice of providing caps on the leads has been effective in preventing damage to the patient during the introduction of the lead. Improper use of the lead, as by unauthorized immediate or premature insertion, can lead to inadequate fixation, possibly resulting in dislodgement or unsatisfactory pacing.
There have been two areas identified by the present inventors where improvements may be made in the use of mannitol caps in the protection of helical leads or securing elements. First, because of the physical shape of the helical element, mannitol present within the core of the helix tends to be dissolved out more slowly than desirable from within the helix and adjacent any electrode at the proximal end of the helical element. Further, any slowly dissolving mannitol that does remain within the confines or central area of the helix may have a tendency to obstruct the advance of the helical element through the tissue until all of the mannitol in the core area has been removed. Second, the lack of consistent rates of dissolution of the caps from the helical element, for example where the lead was prematurely positioned into soft tissue, tends to require surgeons to wait for a maximum length of time to provide assurance of the cap dissolution and proper electrical contact. Any prolongation of implant time, as would be the case here, is highly undesirable. Although neither of these considerations affect the in place performance of the connected leads, the reduction in procedural time by reducing or eliminating these effects is desired.